The packaging industry is continually looking for ways to reduce the amount of material used in a package while improving or maintaining the integrity and functionality of the package. This is of particular importance in the area of beer and beverage containers due to extremely high volumes. With such large volumes, small reductions in the materials used for each package adds up to a very significant savings of money and of metal resources.
One area where a great deal of work has been done to reduce material costs is the generally flat end wall closing of a conventional, generally cylindrical container. As is well known, this end wall, or container end, is less able to withstand internal pressurization of the container than the side wall for a given gauge of metal. Thus, for example, while the industry has been able to reduce the side wall of a two-piece aluminum beer and beverage container to about 0.004" in gauge thickness, the container end is on the order of 0.011" to 0.012", depending on the container end's intended purpose and design. Reduction in the gauge of a container end of a beer or beverage container of a few thousandths of an inch will result in large raw material savings.
The container end typically has a center panel surrounded by a chuck wall which is integrally connected to a peripheral flange or curl. Internal pressurization of the container can cause the center panel on the container end to dome, or bulge, upwardly due to axial upward forces. The axial upward forces acting on the center panel result, in turn, in radially inward forces being applied to the chuck wall which is positioned on the inner surface of the side wall. The curl is provided to double-seam the container end to the container. Thus, the chuck wall may be pulled away from the side wall allowing the center panel to bulge even higher. A variety of problems are encountered if the center panel rises above the double seam of the container. Historically, this has been compensated for by utilizing a relatively thick container end. However, in order to thin the container end, an improved container end design was needed in order to help the container end withstand bulging and buckling forces.
U.S. Pat. No. 3,417,898 to Bozek et. al., discloses an early development in this area. Specifically, Bozek et. al., discloses provision of a countersink area between a center panel of the container end and a peripheral curl used to secure the end to a container. The container end of Bozek et. al., was formed in a two-stage operation. The container end was initially drawn to be relatively deep and to include a side wall (or chuck wall) integral with a peripheral curl. The side wall was integrally connected by a segment having a large radius to a relatively flat recessed central panel. In a subsequent operation, the recessed panel is moved axially upwardly and the segment with the large radius is deformed together with a lower portion of the side wall to form an inner side wall, a sharp bend (typically referred to as a countersink bottom) and an outer side wall. In an alternative embodiment, a third operation is subsequently performed which increases the arc of the bend and causes the inner wall to flare radially outward so that the upper portion of the inner wall is brought into contact with the outer side wall.
Since Bozek et. al., considerable work has been done to improve the buckle strength of a container end through modification of the countersink area usually in concert With other structural elements of the container end. The conventional practice in making a container end today is to start with a shell that includes a countersink portion between the center panel and the curl. The shell is made in a shell press for converting a disk of metal, or cutedge, into a shell. The shell is then processed in a conversion press, where the shell undergoes various operations to be converted to a finished container end. For example, a ring pull or non-detachable tab is attached to the end, and score lines are provided for a pour hole. A container end maker may purchase standard shells from a vendor or operate its own shell presses.
The structural design of a container end can be advantageously used to reduce the material required to produce the container end. Improved strength resulting from an improved structural design will compensate the container end for loss of strength due to reduction in gauge thickness.
Of course, reduction of materials in the container end is limited by the performance required of the packaging. For example, beer and beverage containers must be able to withstand internal pressurization, mechanized seaming and handling processes, and shipping. In large part, the industry has established minimum standards to ensure performance. For example, a container end must be able to withstand 90 to 93 psig internal pressure without permanent deformation such as buckling. These pressure levels may be experienced during pasteurization of the filled product after the container is sealed.
Material reduction by structural redesign modification is also limited by industry requirements. For example, in the beer and beverage container industry, container ends are required to meet certain dimensional specifications so that the ends will fit on containers made by any vendor and so that the ends are compatible with standard equipment for attaching the ends to the containers.
Material reduction by structural redesign is also limited by the fact that it is expensive to shut down and modify the high-speed precision tooling required to make container ends. This limitation is multiplied by the large number of machines required to produce the high volume of containers produced in a year.